Much of the harbour’s foreshore is bordered by sea walls, many of them carved from local sandstone more than a century ago. They have long been a favourite spot for harbour visitors to sit and contemplate our waterways but the character and charm of sea walls isn’t so welcoming to life below the waterline.

“Seawalls are very homogeneous, vertical structures, a bit shaded and they don’t attract the diverse life that you’ll get on a naturally sloping rocky reef with lots of rock pools and crevices,” says marine ecologist Dr Katherine Dafforn.

“They still have things growing on them, but they generally host a very different community.”

Dafforn is studying our underwater architecture through the collaborative Sydney Harbour Research Program, co-ordinated by the Sydney Institute of Marine Science.

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The not-for-profit institute, based at Mosman’s Chowder Bay, relies on public and private-sector funding to undertake key areas of research, including community fundraising through the annual harbour hike (see next page).

Dafforn says it is vital to ensure the harbour’s built structures don’t attract the wrong kind of resident. “Marine urban development is really accelerating and we’re going to see a lot more artificial structures introduced into waterways.”

Researchers have tried to improve the diversity of seawall life in parts of the harbour by placing terracotta flowerpots on walls to provide rockpool-like habitats – and the trials have been promising.

But a few flowerpots are not nearly enough to transform underwater Sydney.

Urban sprawl beneath the waves provides shelter to marine invaders that have hitchhiked into our harbour on travelling ships. Harbour-dwellers that thrive in the crevices of pilings and pontoons are often invasive species hostile to Sydney’s natural ecosystem, and include sea squirts and bristle worms which spread across habitats, driving indigenous marine life away.

Dafforn and her team have recently landed a $380,000 Australian Research Council grant, in partnership with NSW Fisheries and Lend Lease, to study the design of artificial structures in marine environments.

First stop is Barangaroo, where Dafforn will have the chance to test key green marine engineering theories on live building works.

“We’re helping Lend Lease engineer the foreshore development, so that they have a much more ecologically-friendly marine environment underneath,” she says.

“These artificial structures need to be there for coastal protection and infrastructure, but we’ll create multiple uses for them, by transplanting oysters that can filter the water column and help with pollution control, and transplanting threatened seaweeds that will host diverse and healthy marine life.”

Dafforn’s team has started an underwater Neighbourhood Watch, dropping GoPro cameras around various structures beneath the harbour surface.

The waterproof videocams film the secret lives of fish, and provide the ‘before’ shots for the green engineering makeover at Barangaroo.

“We’re getting a baseline of what the conditions are like at Barangaroo and at lots of other sites around the harbor so we’ll know if our engineering designs have worked,” she says.

Dafforn says that green marine engineering is becoming a priority around the world, because climate change is driving more extreme storms and a rise in sea-levels, which in turn means the need for more considered forms of coastal protection.

“Some places are putting up more seawalls and break walls, but that actually multiplies wave energy,” she says.

“Ideally we would release more land back to natural mangroves and salt marsh and break up hard seawalls with tidal rock platforms and beaches which absorb wave energy - but it’s really hard to get cities to retreat from important urban land.”

In the western reaches of the harbour and further upriver, where long-term industrial pollution infuses the sediment, remaining mangrove stands are under constant threat – and seawalls aren’t helping.

Another Sydney Harbour Research Program researcher, marine ecologist Dr Melanie Bishop is studying the impact of sea walls on Sydney Harbour’s riverscapes – where they border an estimated 45 per cent of the Parramatta River.

Seawalls have replaced mud, sea grass beds and mangrove forests that previously divided land and water.

“Along the upper Parramatta River there are still large areas of fringing mangrove along the riverbanks with sea walls built right up to them, constraining their landward extent, so we get much narrower mangrove forests,” Bishop says.

They might be stinky, slimy and spiky on bare feet and an unwelcome obstruction to our much-prized water views, but mangrove forests form low-lying thickets protecting shorelines from storm surges and wind damage, and they are going to be even more critical as extreme weather events become more common.